When the E. coli chromosome is damaged by radiation or chemicals, a new set of cellular functions becomes expressed. These include new pathways for DNA repair, mutagenesis and prophage induction. The long range goal of this research is to analyze the biochemical mechanisms that underlie these changes. Central to the induction process is the recA gene because mutations in this gene block expression of all SOS functions. Accordingly, a more immediate objective of this research is to understand the central role of the recA protein in the induction of SOS functions. Recent experiments by Roberts and others have indicated that the mechanism of induction of prophage lambda is proteolytic cleavage of lambda repressor by the recA protein. Other work has shown that the recA protein is both modified chemically in the induced cell and its rate of synthesis increases dramatically. We have isolated a set of mutations which change the activity of recA protein as a protease or phage repressor as a substract and will employ these to characterize further the cleavage of repressor by the recA protein.